The present invention relates generally to passive infrared detectors and particularly to a protective agent for windows or lenses which are required to be transmissive to infrared radiation and diffuse, absorbent or reflective to visible and/or ultraviolet light, wherein the protective agent prevents a degradation in infrared transmission over time, even when exposed to sunlight.
In a passive infrared detector, infrared radiation is generally focused onto an infrared sensor which provides an output signal indicative of the level of received infrared radiation. It is desired to allow infrared energy to pass through to the infrared sensor while blocking passage thereto of visible or ultraviolet light. Accordingly, passive infrared detectors generally employ lenses or windows made of polyethylene, particularly high density polyethylene (HDPE), since polyethylene is generally transparent to infrared energy in the wavelength range of 5-16 xcexcm. An example of such a passive infrared detector is described in U.S. Pat. No. 4,321,594 to Galvin et al., the disclosure of which is incorporated herein by reference.
Recently pigments have been added to the polyethylene window or lens, or pigmented or colored filters have been placed between the window or lens and the infrared sensor, or mirrors have been coated with pigments. One reason, inter alia, for the pigment addition is that it provides the passive detectors with an aesthetic appearance and gives the window or lens a similar hue as the detector housing. Another reason is that the bare polyethylene is prone to passage therethrough of certain kinds of spurious visible light, such as light from halogen lamps, for example from the headlights of a passing vehicle. The pigment addition can help prevent the spurious light from passing through to the infrared sensor and thus prevent false alarms. Several standards have been adopted which set the engineering requirements for such pigment additions (for prevention of false alarms) including European Standard EN 50131-2-2 (1993), Section 9.5.4, International Standard IEC 839-2-6-1990, Section 6.2.5, British Standard LPS 1169 Issue 2 (June 1992) Section 8.2.9, and Canadian Standard ULC-S-306-M89 Section 8.8.6.
The pigment addition ideally should diffuse, reflect or absorb light in the visible and/or UV range, and thereby prevent passage therethrough to the infrared sensor, while at the same time allow passage therethrough of infrared radiation. In practice, pigments are selected which have the above electromagnetic wave transmissive charateristics, and/or have a particle size that diffuses visible light while passing through infrared radiation.
It is desirable to mix the pigment with the raw polyethylene material. The pigment should not cause a degradation in the transmissive or mechanical properties of the polyethylene. Zinc sulfide (ZnS) has been found to be a particularly suitable pigment which provides a satisfactory combination of diffusion of visible and ultraviolet light and unobstructed transmission of infrared radiation, and which can be mixed with the raw polyethylene material.
However, a problem exists with ZnS and similar pigments in that over time, particularly with exposure to sunlight, the pigment reacts with polyethylene and creates reaction products which absorb infrared radiation in the wavelength range of 5-16 xcexcm. (For example, in the case of ZnS, an undesirable infrared absorption band develops over time in the range of about 6-10 xcexcm.) These reaction products reduce the amount of infrared radiation which reaches the sensor and thus cause a degradation in the passive detector performance.
The present invention seeks to solve the abovementioned problem of the prior art. Specifically, the present invention provides a protective agent for windows, filters or lenses which are required to be transmissive to infrared radiation and diffuse, absorbent or reflective to visible and/or ultraviolet light, wherein the protective agent prevents a degradation in infrared transmission over time, even when exposed to sunlight.
There is thus provided in accordance with a preferred embodiment of the present invention an optical element including a substrate which is generally transparent to infrared radiation in a wavelength range of 5-16 xcexcm, a pigment disposed in the substrate in an amount that does not generally decrease transmission of the infrared radiation, the pigment being generally non-transmissive to at least one of visible and ultraviolet light, the pigment being reactable with the substrate over time to create a reaction product which can decrease transmission of the infrared radiation, and a protective agent disposed in the substrate in an amount that does not generally decrease transmission of the infrared radiation and which generally prevents creation of the reaction product which can decrease transmission of the infrared radiation. The optical element may be a lens for a passive infrared detector assembly, for example.
In accordance with a preferred embodiment of the present invention the protective agent is disposed in the substrate or mixed with the pigment.
In accordance with an alternatively preferred embodiment of the present invention the pigment is coated with the protective agent such that the pigment generally does not contact the substrate.
Further in accordance with a preferred embodiment of the present invention the pigment has a property of oxidizing the substrate so as to create an oxidation product which can decrease transmission of the infrared radiation, and the protective agent has a property of reacting with the pigment so as to prevent the pigment from oxidizing the substrate, and wherein the protective agent is mixed with the substrate and the pigment in an amount sufficient to generally prevent oxidation of the substrate by the pigment.
Still further in accordance with a preferred embodiment of the present invention the protective agent is pigmentary. Alternatively, the protective agent may be non-pigmentary.
In accordance with a preferred embodiment of the present invention the substrate includes polyethylene or high density polyethylene.
Additionally in accordance with a preferred embodiment of the present invention the pigment includes zinc sulfide.
Further in accordance with a preferred embodiment of the present invention the protective agent includes zinc oxide (ZnO). It is noted that although ZnO is less efficient than ZnS as an opacifier of polyethylene, ZnO is a stronger ultraviolet absorber and has been presumed to dissipate much of the absorbed energy thermally (see J. A. Manasso and W. S. Castor, Jr., xe2x80x9cAdditives for Plasticxe2x80x9d, Vol. 2, Raymond B. Semor Ed., Academic Press, 1978, p. 93).
In accordance with a preferred embodiment of the present invention the amount of the pigment relative to the amount of the protective agent is in a range between 1:4 and 4:1 inclusive.
Further in accordance with a preferred embodiment of the present invention the pigment and the protective agent include particles in a size range of 0.5-6 xcexcm.
There is also provided in accordance with a preferred embodiment of the present invention a passive infrared detector assembly including an optical filter including a substrate which is generally transparent to infrared radiation in a wavelength range of 5-16 xcexcm, a pigment disposed in the substrate in an amount that does not generally decrease transmission of the infrared radiation, the pigment being generally non-transmissive to at least one of visible and ultraviolet light, the pigment being reactable with the substrate over time to create a reaction product which can decrease transmission of the infrared radiation, and a protective agent disposed in the substrate an amount that does not generally decrease transmission of the infrared radiation and which generally prevents creation of the reaction product which can decrease transmission of the infrared radiation, an infrared sensor positioned relative to the substrate such that infrared radiation can impinge upon the sensor after passing through the optical element, the sensor providing an output signal indicative of received infrared radiation, and alarm apparatus operatively connected to the sensor which produces an alarm signal based upon the output signal of the sensor.